Dynamic management of programming states to improve endurance

ABSTRACT

A storage device with a memory may include improved endurance and programming speed by modifying the programming states of the memory blocks. For example, the blocks may be three bit memory blocks, but a dynamic reassignment of verify levels and read margins can result in the block acting like a two bit memory block. Memory blocks may be designed for a certain number of bits per cell (i.e. number of states) and the programming is based on that number. However, single level cell (SLC) programming is still possible in addition to programming according to the number of bits per cell that the memory is designed for. Multiple SLC programming steps can be used to modify the number of states for certain memory cells by the memory controller.

TECHNICAL FIELD

This application relates generally to memory devices. More specifically,this application relates to improving endurance and faster programmingby reassigning programming states.

BACKGROUND

Non-volatile memory systems, such as flash memory, have been widelyadopted for use in consumer products. Flash memory may be found indifferent forms, for example in the form of a portable memory card thatcan be carried between host devices or as a solid state disk (SSD)embedded in a host device. NAND is one example of a non-volatile flashmemory. Flash memory may have a limited endurance, which determines anumber of times a particular memory can be erased and re-programmed. Asthe non-volatile memory cell scales to smaller dimensions with highercapacity per unit area, the cell endurance due to program and erasecycling, and disturbances (e.g. due to either read or program) maybecome more prominent. The overall vulnerability of memory cells and thedefect level during the silicon process may become elevated as the celldimension shrinks and process complexity increases, which directlyaffects the data integrity at both the memory and system. For example,with an increase in the number of bits per cell, the bit error rate mayincrease as both programming and reading must be more exact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an example non-volatile memory system.

FIG. 1B is a block diagram of a storage module that includes a pluralityof non-volatile memory systems.

FIG. 1C is a block diagram of a hierarchical storage system.

FIG. 2A is a block diagram of exemplary components of a controller of anon-volatile memory system.

FIG. 2B is a block diagram of exemplary components of a non-volatilememory of a non-volatile memory storage system.

FIG. 3 is a diagram illustrating charge levels in a multi-level cellmemory operated to store two bits of data in a memory cell.

FIG. 4 is a diagram illustrating charge levels in a multi-level cellmemory

FIG. 5 is a diagram of charge distribution with single level cellprogramming.

FIG. 6 is a diagram of charge distribution showing multiple single levelcell programming.

FIG. 7 is another diagram of charge distribution showing multiple singlelevel cell programming.

FIG. 8 is a flowchart illustrating program state modification.

DESCRIPTION OF THE EMBODIMENTS

A storage device with a memory may include improved endurance andprogramming speed by modifying the programming states of the memoryblocks. For example, the blocks may be three bit memory blocks, but adynamic reassignment of read margins and verify levels can result in theblock acting like a two bit memory block. Memory blocks may be designedfor a certain number of bits per cell (i.e. number of states) and theprogramming is based on that number. However, single level cell (SLC)programming is also possible in addition to programming according to thenumber of bits per cell that the memory is designed for. Multiple SLCprogramming steps can be used to modify the number of states for certainmemory cells.

This dynamic reassignment of the programed state may be performed whenan erase cycle is needed. The original data and newer data can beprogrammed on the same SLC block and the erase can be performed once(e.g. the erase is every two programs rather than one program). As aresult, the number of program/erase (“P/E”) cycling can be doubled whilethe SLC is converted to a different type of memory (different number ofstates) that may have better endurance and faster programming.Decreasing the number of states for each memory cell may result in theP/E ratio being increased. The increase of the block programming allowedper erase instance may help increase the cycling performance of theflash memory, mainly due to the reduced stress time from the erase.Twice as many programming operations can be carried out and thus the TBW(total byte written) to the system can be increased. This system/methodsmartly manages the data written to the flash memory during the programand erase operation to the memory blocks. By doing so, the erase countsto the memory may be effectively reduced while the performance of thewrite can be enhanced.

In one example, the memory device may be an 8 states or 3 bit per cellmemory. The circuitry of the memory system can read/write with the 8states, but there is no 4 state modes for this memory. The circuitry canalso read/write to 2 states using SLC programming. Regardless of thenumber of states that the memory is designed for, it can still performSLC programming with 2 states. Using multiple SLC programming steps andmodifying both the read margin and program verify level the 8-statememory can be used as 4-state memory. This provides interchangeabilityfor different technologies that were previously incompatible. In otherexamples, the number of states (or bits per cell) may be different, butSLC may still be used to treat the memory as if it was designed for adifferent number of states.

FIGS. 1A-2B are exemplary memory systems which may implement BDRL. FIG.1A is a block diagram illustrating a non-volatile memory system. Thenon-volatile memory system 100 includes a controller 102 andnon-volatile memory that may be made up of one or more non-volatilememory die 104. As used herein, the term die refers to the set ofnon-volatile memory cells, and associated circuitry for managing thephysical operation of those non-volatile memory cells, that are formedon a single semiconductor substrate. Controller 102 interfaces with ahost system and transmits command sequences for read, program, and eraseoperations to non-volatile memory die 104. The non-volatile memory die104 may store an operating system for the host.

Examples of host systems include, but are not limited to, personalcomputers (PCs), such as desktop or laptop and other portable computers,tablets, mobile devices, cellular telephones, smartphones, personaldigital assistants (PDAs), gaming devices, digital still cameras,digital movie cameras, and portable media players. For portable memorycard applications, a host may include a built-in receptacle for one ormore types of memory cards or flash drives, or a host may requireadapters into which a memory card is plugged. The memory system mayinclude its own memory controller and drivers but there may also be somememory-only systems that are instead controlled by software executed bythe host to which the memory is connected. In some memory systemscontaining the controller, especially those embedded within a host, thememory, controller and drivers are often formed on a single integratedcircuit chip. The host may communicate with the memory card using anycommunication protocol such as but not limited to Secure Digital (SD)protocol, Memory Stick (MS) protocol and Universal Serial Bus (USB)protocol.

The controller 102 (which may be a flash memory controller) can take theform of processing circuitry, a microprocessor or processor, and acomputer-readable medium that stores computer-readable program code(e.g., software or firmware) executable by the (micro)processor, logicgates, switches, an application specific integrated circuit (ASIC), aprogrammable logic controller, and an embedded microcontroller, forexample. The controller 102 can be configured with hardware and/orfirmware to perform the various functions described below and shown inthe flow diagrams. Also, some of the components shown as being internalto the controller can also be stored external to the controller, andother components can be used. Additionally, the phrase “operatively incommunication with” could mean directly in communication with orindirectly (wired or wireless) in communication with through one or morecomponents, which may or may not be shown or described herein.

As used herein, a flash memory controller is a device that manages datastored on flash memory and communicates with a host, such as a computeror electronic device. A flash memory controller can have variousfunctionality in addition to the specific functionality describedherein. For example, the flash memory controller can format the flashmemory to ensure the memory is operating properly, map out bad flashmemory cells, and allocate spare cells to be substituted for futurefailed cells. Some part of the spare cells can be used to hold firmwareto operate the flash memory controller and implement other features. Inoperation, when a host needs to read data from or write data to theflash memory, it will communicate with the flash memory controller. Ifthe host provides a logical address to which data is to be read/written,the flash memory controller can convert the logical address receivedfrom the host to a physical address in the flash memory. (Alternatively,the host can provide the physical address). The flash memory controllercan also perform various memory management functions, such as, but notlimited to, wear leveling (distributing writes to avoid wearing outspecific blocks of memory that would otherwise be repeatedly written to)and garbage collection (after a block is full, moving only the validpages of data to a new block, so the full block can be erased andreused).

Non-volatile memory die 104 may include any suitable non-volatilestorage medium, including NAND flash memory cells and/or NOR flashmemory cells. The memory cells can take the form of solid-state (e.g.,flash) memory cells and can be one-time programmable, few-timeprogrammable, or many-time programmable. The memory cells can also besingle-level cells (SLC), multiple-level cells (MLC), triple-level cells(TLC), or use other memory cell level technologies, now known or laterdeveloped. Also, the memory cells can be fabricated in a two-dimensionalor three-dimensional fashion.

The interface between controller 102 and non-volatile memory die 104 maybe any suitable flash interface, such as Toggle Mode 200, 400, or 800.In one embodiment, memory system 100 may be a card based system, such asa secure digital (SD) or a micro secure digital (micro-SD) card. In analternate embodiment, memory system 100 may be part of an embeddedmemory system. For example, the flash memory may be embedded within thehost, such as in the form of a solid state disk (SSD) drive installed ina personal computer.

Although in the example illustrated in FIG. 1A, non-volatile memorysystem 100 includes a single channel between controller 102 andnon-volatile memory die 104, the subject matter described herein is notlimited to having a single memory channel. For example, in some NANDmemory system architectures, such as in FIGS. 1B and 1C, 2, 4, 8 or moreNAND channels may exist between the controller and the NAND memorydevice, depending on controller capabilities. In any of the embodimentsdescribed herein, more than a single channel may exist between thecontroller and the memory die, even if a single channel is shown in thedrawings.

FIG. 1B illustrates a storage module 200 that includes pluralnon-volatile memory systems 100. As such, storage module 200 may includea storage controller 202 that interfaces with a host and with storagesystem 204, which includes a plurality of non-volatile memory systems100. The interface between storage controller 202 and non-volatilememory systems 100 may be a bus interface, such as a serial advancedtechnology attachment (SATA) or peripheral component interface express(PCIe) interface. Storage module 200, in one embodiment, may be a solidstate drive (SSD), such as found in portable computing devices, such aslaptop computers, and tablet computers.

FIG. 1C is a block diagram illustrating a hierarchical storage system. Ahierarchical storage system 210 includes a plurality of storagecontrollers 202, each of which control a respective storage system 204.Host systems 212 may access memories within the hierarchical storagesystem via a bus interface. In one embodiment, the bus interface may bea non-volatile memory express (NVMe) or a fiber channel over Ethernet(FCoE) interface. In one embodiment, the system illustrated in FIG. 1Cmay be a rack mountable mass storage system that is accessible bymultiple host computers, such as would be found in a data center orother location where mass storage is needed.

FIG. 2A is a block diagram illustrating exemplary components ofcontroller 102 in more detail. Controller 102 includes a front endmodule 108 that interfaces with a host, a back end module 110 thatinterfaces with the one or more non-volatile memory die 104, and variousother modules that perform functions which will now be described indetail.

A module may take the form of a packaged functional hardware unitdesigned for use with other components, a portion of a program code(e.g., software or firmware) executable by a (micro)processor orprocessing circuitry that usually performs a particular function ofrelated functions, or a self-contained hardware or software componentthat interfaces with a larger system, for example. For example, eachmodule may include an application specific integrated circuit (ASIC), aField Programmable Gate Array (FPGA), a circuit, a digital logiccircuit, an analog circuit, a combination of discrete circuits, gates,or any other type of hardware or combination thereof. Alternatively orin addition, each module may include memory hardware, such as a portionof the memory 104, for example, that comprises instructions executablewith a processor to implement one or more of the features of the module.When any one of the modules includes the portion of the memory thatcomprises instructions executable with the processor, the module may ormay not include the processor. In some examples, each module may just bethe portion of the memory 104 or other physical memory that comprisesinstructions executable with the processor to implement the features ofthe corresponding module.

The controller 102 may include program state circuitry 112. The programstate circuitry 112 may be used for assigning a different number ofstates to cells in a block. That number may be different from what theblock was designed for. As described, SLC programming may be used togenerate a different distribution of states. For example, the memory 104may be designed for 8 states per cell (3 bits per cell), but using SLCprogramming, certain blocks may be programmed with only 4 states (2 bitsper cell) even if the other circuitry of the NAND or memory system wasonly designed for 8 states per cell.

Referring again to modules of the controller 102, a buffer manager/buscontroller 114 manages buffers in random access memory (RAM) 116 andcontrols the internal bus arbitration of controller 102. A read onlymemory (ROM) 118 stores system boot code. Although illustrated in FIG.2A as located separately from the controller 102, in other embodimentsone or both of the RAM 116 and ROM 118 may be located within thecontroller. In yet other embodiments, portions of RAM and ROM may belocated both within the controller 102 and outside the controller.Further, in some implementations, the controller 102, RAM 116, and ROM118 may be located on separate semiconductor die.

Front end module 108 includes a host interface 120 and a physical layerinterface (PHY) 122 that provide the electrical interface with the hostor next level storage controller. The choice of the type of hostinterface 120 can depend on the type of memory being used. Examples ofhost interfaces 120 include, but are not limited to, SATA, SATA Express,SAS, Fibre Channel, USB, PCIe, and NVMe. The host interface 120typically facilitates transfer for data, control signals, and timingsignals.

Back end module 110 includes an error correction controller (ECC) engine124 that encodes the data bytes received from the host, and decodes anderror corrects the data bytes read from the non-volatile memory. Acommand sequencer 126 generates command sequences, such as program anderase command sequences, to be transmitted to non-volatile memory die104. A RAID (Redundant Array of Independent Drives) module 128 managesgeneration of RAID parity and recovery of failed data. The RAID paritymay be used as an additional level of integrity protection for the databeing written into the non-volatile memory system 100. In some cases,the RAID module 128 may be a part of the ECC engine 124. A memoryinterface 130 provides the command sequences to non-volatile memory die104 and receives status information from non-volatile memory die 104. Inone embodiment, memory interface 130 may be a double data rate (DDR)interface, such as a Toggle Mode 200, 400, or 800 interface. A flashcontrol layer 132 controls the overall operation of back end module 110.

Additional components of system 100 illustrated in Figure. 2A includemedia management layer 138, which performs wear leveling of memory cellsof non-volatile memory die 104. System 100 also includes other discretecomponents 140, such as external electrical interfaces, external RAM,resistors, capacitors, or other components that may interface withcontroller 102. In alternative embodiments, one or more of the physicallayer interface 122, RAID module 128, media management layer 138 andbuffer management/bus controller 114 are optional components that arenot necessary in the controller 102.

The FTL or MML 138 may be integrated as part of the flash managementthat may handle flash errors and interfacing with the host. Inparticular, MML may be a module in flash management and may beresponsible for the internals of NAND management. In particular, the MML138 may include an algorithm in the memory device firmware whichtranslates writes from the host into writes to the flash memory 104. TheMML 138 may be needed because: 1) the flash memory may have limitedendurance; 2) the flash memory 104 may only be written in multiples ofpages; and/or 3) the flash memory 104 may not be written unless it iserased as a block. The MML 138 understands these potential limitationsof the flash memory 104 which may not be visible to the host.Accordingly, the MML 138 attempts to translate the writes from host intowrites into the flash memory 104. As described below, erratic bits maybe identified and recorded using the MML 138. This recording of erraticbits can be used for evaluating the health of blocks.

FIG. 2B is a block diagram illustrating exemplary components ofnon-volatile memory die 104 in more detail. Non-volatile memory die 104includes peripheral circuitry 141 and non-volatile memory array 142.Non-volatile memory array 142 includes the non-volatile memory cellsused to store data. The non-volatile memory cells may be any suitablenon-volatile memory cells, including NAND flash memory cells and/or NORflash memory cells in a two dimensional and/or three dimensionalconfiguration. Peripheral circuitry 141 includes a state machine 152that provides status information to controller 102. Non-volatile memorydie 104 further includes a data cache 156 that caches data. Exemplaryperipheral circuitry 141 may include clocks, pumps, ESD, current shunt,current sink, and/or closely-packed logic circuits.

The logic interpretation of the program state is achieved by the NANDread circuits by setting appropriate read levels against the expectedcharge states. The data into and from the NAND chip may be controlled bythe MCU or ASIC circuitry and a data processing algorithm unit of thesystem where the data is interpreted and further transferred to thehost.

FIG. 3 is a diagram illustrating charge levels in cell memory. Thecharge storage elements of the memory cells are most commonly conductivefloating gates but may alternatively be non-conductive dielectric chargetrapping material. Each cell or memory unit may store a certain numberof bits of data per cell. In FIG. 3, MLC memory may store four statesand can retain two bits of data: 00 or 01 and 10 or 11. Alternatively,MLC memory may store eight states for retaining three bits of data asshown in FIG. 4. In other embodiments, there may be a different numberof bits per cell.

The right side of FIG. 3 illustrates a memory cell that is operated tostore two bits of data. This memory scheme may be referred to as eX2memory because it has two bits per cell. The memory cells may beoperated to store two levels of charge so that a single bit of data isstored in each cell. This is typically referred to as a binary or singlelevel cell (SLC) memory. SLC memory may store two states: 0 or 1.Alternatively, the memory cells may be operated to store more than twodetectable levels of charge in each charge storage element or region,thereby to store more than one bit of data in each. This latterconfiguration is referred to as multi-level cell (MLC) memory. FIG. 3illustrates a two-bit per cell memory scheme in which either four states(Erase, A, B, C) or with two states of SLC memory. This two-bit per cellmemory (i.e. eX2) memory can operate as SLC or as four state MLC.Likewise, as described with respect to FIG. 4, three-bit per cell memory(i.e. eX3) can operate either as SLC or as eight state MLC. The NANDcircuitry may be configured for only a certain number of bit per cellMLC memory, but still operate as SLC. In other words, MLC memory canoperate as a MLC or SLC, but with regard to the MLC operation three bitper cell memory cannot operate as two bit per cell memory andvice-versa. The embodiments described below utilize any MLC memoryscheme's ability to work with SLC to then operate at different bits percell.

FIG. 3 illustrates one implementation of the four charge levels used torepresent two bits of data in a memory cell. In implementations of MLCmemory operated to store two bits of data in each memory cell, eachmemory cell is configured to store four levels of charge correspondingto values of “11,” “01,” “10,” and “00.” Each bit of the two bits ofdata may represent a page bit of a lower page or a page bit of an upperpage, where the lower page and upper page span across a series of memorycells sharing a common word line. Typically, the less significant bit ofthe two bits of data represents a page bit of a lower page and the moresignificant bit of the two bits of data represents a page bit of anupper page. The read margins are established for identifying each state.The three read margins (AR, BR, CR) delineate the four states. Likewise,there is a verify level (i.e. a voltage level) for establishing thelower bound for programming each state.

FIG. 3 is labeled as LM mode which may be referred to as lower at middlemode and will further be described below regarding the lower at middleor lower-middle intermediate state. The LM intermediate state may alsobe referred to as a lower page programmed stage. A value of “11”corresponds to an un-programmed state or erase state of the memory cell.When programming pulses are applied to the memory cell to program a pagebit of the lower page, the level of charge is increased to represent avalue of “10” corresponding to a programmed state of the page bit of thelower page. The lower page may be considered a logical concept thatrepresents a location on a multi-level cell (MLC). If the MLC is twobits per cell, a logical page may include all the least significant bitsof the cells on the wordline that are grouped together. In other words,the lower page is the least significant bits. For a page bit of an upperpage, when the page bit of the lower page is programmed (a value of“10”), programming pulses are applied to the memory cell for the pagebit of the upper page to increase the level of charge to correspond to avalue of “00” or “10” depending on the desired value of the page bit ofthe upper page. However, if the page bit of the lower page is notprogrammed such that the memory cell is in an un-programmed state (avalue of “11”), applying programming pulses to the memory cell toprogram the page bit of the upper page increases the level of charge torepresent a value of “01” corresponding to a programmed state of thepage bit of the upper page.

FIG. 4 is a diagram illustrating charge levels in a multi-level cellmemory operated to store three bits of data in a memory cell. FIG. 4illustrates MLC memory with three bits of data which are stored in asingle cell by establishing eight states or voltage level distinctions.This memory may be referred to as X3 memory. FIG. 4 illustrates thestages that may be used for programming three bit memory. In a firststage, the voltage levels are divided out at two levels, and at thesecond stage (i.e. foggy program), those two levels are divided up intothe eight states without setting the distinct levels between states. Atthe third stage (i.e. fine program), the voltage levels for each of theeight states are separated and distinct. The fine programmingestablishes the voltage levels for each of the states. As compared withtwo bit memory, the three bit memory in FIG. 4 requires more exactprogramming voltages to avoid errors. Electron movement or loss from thecharge values may result in problems. Endurance and programming speedmay decrease based on the exact programming that is required.

In alternative embodiments, there may be memory schemes with increasedbits per cell (e.g. 4 bits per cell or X4 memory). Each of those memoryschemes may operate using that number of bits per cell (e.g. “n” bitsper cell where n is an integer of 2 or more), but also by using SLCprogramming. Accordingly, the system and methods described herein willallow operation under n bits per cell or using SLC programming to actlike a different bit per cell memory (e.g. any number less than n).

FIG. 5 is a diagram of charge distribution with single level cell (SLC)programming. The two states are the erase state and the program state.This particular cell can provide either a 0 or a 1 as the stored bit.FIG. 5 illustrates the voltage along the x-axis with both SLC states.The distributions represent the range of voltage levels for each state.The programming performed for these two states is SLC programming. Evenfor memory that is multiple bits per cell, this type of SLC programmingmay still be performed. In other words, multiple bit per cell memory canstill be used with two states from SLC programming. This may be thefirst programming after a fresh erase.

FIG. 6 is a diagram of charge distribution showing multiple single levelcell programming. In one embodiment, the programming shown in both FIG.6 and FIG. 7 is part of a second programming that follows the initialprogramming of FIG. 5. Once the block is fully programmed, the new datamay be received for programming. As described with respect to FIG. 8,there may be a flag that is set to identify whether the block is beingutilized for a different scheme. In particular, this flag may identifywhether the block is fully programmed (e.g. 3 bit per cell memory beingprogrammed with 3 bits per cell) or partially programmed using multipleSLC programs (e.g. 3 bit per cell memory being programmed with 2 bitsper cell). The secondary programming shown with respect to FIG. 6 andFIG. 7 may only be performed when the flag identifies this partialprogramming.

FIG. 6 illustrates that the program state (from FIG. 5) can bereprogrammed to be another program state. The two programmed statesshown in FIG. 6 are the B state (SLC_B) and the C state (SLC_C). The Bstate is the original state programmed with the first SLC programmingoperation and the C state is the newly programmed state. The C state isprogrammed from the original program state (from FIG. 5) which is nowthe B state. After programming the read levels are adjusted todistinguish between the states. The program verify voltage (PGMV) of theC state is set to be outside of the read level. The reassignment of theread margin and program verify voltages can be used to establish adifferent number of states.

FIG. 7 is another diagram of charge distribution showing multiple singlelevel cell programming. The programming in FIG. 7 may be additionalprogramming or may be part of (e.g. parallel to) the programming shownin FIG. 6. The erase state is programmed to be the A state (SLC_A). TheA state is newly programmed from the original erase state. The thirdprogram verify level (PGMV) is set to be outside of the erase state.Accordingly, the read margin separates the erase state from the A state.Likewise, the read margin in front of the B state is outside of the Astate. In an alternative embodiment, the C state may be programmed basedon the erase state. In other words, the original erase state is used forprogramming the new state C. The embodiment shown in FIG. 6 illustratesthat state C is programmed from the original program state (now state B)rather than the erase state.

FIG. 7 illustrates the establishment of four states (e.g. 2 bits percell). The reassignment of the programmed states may allow memoryconfigured for 8 states (e.g. 3 bits per cell) to be repurposed intofour states using the SLC programming described. This may be referred toas dynamic reassignment of the programmed states or of the distribution.The initial programming (e.g. FIG. 5) may refer to old data that isassigned the lower page, while the new data of the subsequentprogramming is assigned to the upper page.

The assignment of a different bit per cell scheme may be done at theblock level. For example, in a 3 bit per cell NAND, there may be certainblocks that are treated as 2 bit per cell. Alternatively, the managementof the bit per cell structure may be at a larger scale (e.g. meta-block,plane, die, etc.) or at a smaller scale (e.g. page level) in otherembodiments.

FIG. 8 is a flowchart illustrating program state modification. Theprogramming is started in block 802. The programming flag is read inblock 804. The programming flag may indicate whether the memory is in adefault mode or in a mode where the number of states has been modifiedwith SLC programs. In block 806, the memory block is checked to see ifit is freshly erased or partially programmed for the first time. Thememory block is programmed using a first set of verify and read settingsin block 808. The first set of verify and read settings correspond to afirst programming (i.e. FIG. 5). Conversely, a second set of verify andread settings correspond to a second programming (i.e. FIG. 6 and/orFIG. 7). When the block is fully programmed in block 810, theprogramming flag is set for regular programming (default mode) in block812. The read can be continued in block 814. In block 810, fi the blockis not fully programmed, then the first programming continues with theoriginal settings in block 816.

Referring back to block 806, when the block is not freshly erased orpartially programmed, the second programming is performed in block 818.Then a second set of verify and read settings are utilized for thesecond programming in block 820. The second programming is continuedwith the second settings until it is fully programmed in block 822. Theprogramming flag is set for the revised or reassigned (second)programming state mode in block 824. The reading is continued with thesecond set of read settings in block 826.

When the erase process is started in block 828, the erase command isissued in block 830. The programming flag is checked to determine if theflag identifies the first programming or the second programming in block832. When the programing flag identifies the second programming then theblock is erased and the flag is reset in block 834. If the flagidentifies the first programming, then the post first programming flowis continued in block 836. In particular, the first programming isperformed in block 818.

The methods, devices, processing, circuitry, and logic described abovemay be implemented in many different ways and in many differentcombinations of hardware and software. For example, all or parts of theimplementations may be circuitry that includes an instruction processor,such as a Central Processing Unit (CPU), microcontroller, or amicroprocessor; or as an Application Specific Integrated Circuit (ASIC),Programmable Logic Device (PLD), or Field Programmable Gate Array(FPGA); or as circuitry that includes discrete logic or other circuitcomponents, including analog circuit components, digital circuitcomponents or both; or any combination thereof. The circuitry mayinclude discrete interconnected hardware components or may be combinedon a single integrated circuit die, distributed among multipleintegrated circuit dies, or implemented in a Multiple Chip Module (MCM)of multiple integrated circuit dies in a common package, as examples.

Accordingly, the circuitry may store or access instructions forexecution, or may implement its functionality in hardware alone. Theinstructions may be stored in a tangible storage medium that is otherthan a transitory signal, such as a flash memory, a Random Access Memory(RAM), a Read Only Memory (ROM), an Erasable Programmable Read OnlyMemory (EPROM); or on a magnetic or optical disc, such as a Compact DiscRead Only Memory (CDROM), Hard Disk Drive (HDD), or other magnetic oroptical disk; or in or on another machine-readable medium. A product,such as a computer program product, may include a storage medium andinstructions stored in or on the medium, and the instructions whenexecuted by the circuitry in a device may cause the device to implementany of the processing described above or illustrated in the drawings.

The implementations may be distributed. For instance, the circuitry mayinclude multiple distinct system components, such as multiple processorsand memories, and may span multiple distributed processing systems.Parameters, databases, and other data structures may be separatelystored and managed, may be incorporated into a single memory ordatabase, may be logically and physically organized in many differentways, and may be implemented in many different ways. Exampleimplementations include linked lists, program variables, hash tables,arrays, records (e.g., database records), objects, and implicit storagemechanisms. Instructions may form parts (e.g., subroutines or other codesections) of a single program, may form multiple separate programs, maybe distributed across multiple memories and processors, and may beimplemented in many different ways. Example implementations includestand-alone programs, and as part of a library, such as a shared librarylike a Dynamic Link Library (DLL). The library, for example, may containshared data and one or more shared programs that include instructionsthat perform any of the processing described above or illustrated in thedrawings, when executed by the circuitry.

The semiconductor memory elements located within and/or over a substratemay be arranged in two or three dimensions, such as a two dimensionalmemory structure or a three dimensional memory structure. In a twodimensional memory structure, the semiconductor memory elements arearranged in a single plane or a single memory device level. Typically,in a two dimensional memory structure, memory elements are arranged in aplane (e.g., in an x-z direction plane) which extends substantiallyparallel to a major surface of a substrate that supports the memoryelements. The substrate may be a wafer over or in which the layer of thememory elements are formed or it may be a carrier substrate which isattached to the memory elements after they are formed. As a non-limitingexample, the substrate may include a semiconductor such as silicon.

The memory elements may be arranged in the single memory device level inan ordered array, such as in a plurality of rows and/or columns.However, the memory elements may be arrayed in non-regular ornon-orthogonal configurations. The memory elements may each have two ormore electrodes or contact lines, such as bit lines and word lines.

A three dimensional memory array is arranged so that memory elementsoccupy multiple planes or multiple memory device levels, thereby forminga structure in three dimensions (i.e., in the x, y and z directions,where the y direction is substantially perpendicular and the x and zdirections are substantially parallel to the major surface of thesubstrate). As a non-limiting example, a three dimensional memorystructure may be vertically arranged as a stack of multiple twodimensional memory device levels. As another non-limiting example, athree dimensional memory array may be arranged as multiple verticalcolumns (e.g., columns extending substantially perpendicular to themajor surface of the substrate, i.e., in the y direction) with eachcolumn having multiple memory elements in each column. The columns maybe arranged in a two dimensional configuration, e.g., in an x-z plane,resulting in a three dimensional arrangement of memory elements withelements on multiple vertically stacked memory planes. Otherconfigurations of memory elements in three dimensions can alsoconstitute a three dimensional memory array.

By way of non-limiting example, in a three dimensional NAND memoryarray, the memory elements may be coupled together to form a NAND stringwithin a single horizontal (e.g., x-z) memory device levels.Alternatively, the memory elements may be coupled together to form avertical NAND string that traverses across multiple horizontal memorydevice levels. Other three dimensional configurations can be envisionedwherein some NAND strings contain memory elements in a single memorylevel while other strings contain memory elements which span throughmultiple memory levels. Three dimensional memory arrays may also bedesigned in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic three dimensional memory array, one or morememory device levels are formed above a single substrate. Optionally,the monolithic three dimensional memory array may also have one or morememory layers at least partially within the single substrate. As anon-limiting example, the substrate may include a semiconductor such assilicon. In a monolithic three dimensional array, the layersconstituting each memory device level of the array are typically formedon the layers of the underlying memory device levels of the array.However, layers of adjacent memory device levels of a monolithic threedimensional memory array may be shared or have intervening layersbetween memory device levels.

Then again, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device having multiplelayers of memory. For example, non-monolithic stacked memories can beconstructed by forming memory levels on separate substrates and thenstacking the memory levels atop each other. The substrates may bethinned or removed from the memory device levels before stacking, but asthe memory device levels are initially formed over separate substrates,the resulting memory arrays are not monolithic three dimensional memoryarrays. Further, multiple two dimensional memory arrays or threedimensional memory arrays (monolithic or non-monolithic) may be formedon separate chips and then packaged together to form a stacked-chipmemory device.

Associated circuitry is typically required for operation of the memoryelements and for communication with the memory elements. As non-limitingexamples, memory devices may have circuitry used for controlling anddriving memory elements to accomplish functions such as programming andreading. This associated circuitry may be on the same substrate as thememory elements and/or on a separate substrate. For example, acontroller for memory read-write operations may be located on a separatecontroller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this invention is notlimited to the two dimensional and three dimensional exemplarystructures described but cover all relevant memory structures within thespirit and scope of the invention as described herein and as understoodby one of skill in the art.

In the present application, semiconductor memory devices such as thosedescribed in the present application may include volatile memorydevices, such as dynamic random access memory (“DRAM”) or static randomaccess memory (“SRAM”) devices, non-volatile memory devices, such asresistive random access memory (“ReRAM”), electrically erasableprogrammable read only memory (“EEPROM”), flash memory (which can alsobe considered a subset of EEPROM), ferroelectric random access memory(“FRAM”), and magneto-resistive random access memory (“MRAM”), and othersemiconductor elements capable of storing information. Each type ofmemory device may have different configurations. For example, flashmemory devices may be configured in a NAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, inany combinations. By way of non-limiting example, passive semiconductormemory elements include ReRAM device elements, which in some embodimentsinclude a resistivity switching storage element, such as an anti-fuse,phase change material, etc., and optionally a steering element, such asa diode, etc. Further by way of non-limiting example, activesemiconductor memory elements include EEPROM and flash memory deviceelements, which in some embodiments include elements containing a chargestorage region, such as a floating gate, conductive nanoparticles, or acharge storage dielectric material.

Multiple memory elements may be configured so that they are connected inseries or so that each element is individually accessible. By way ofnon-limiting example, flash memory devices in a NAND configuration (NANDmemory) typically contain memory elements connected in series. A NANDmemory array may be configured so that the array is composed of multiplestrings of memory in which a string is composed of multiple memoryelements sharing a single bit line and accessed as a group.Alternatively, memory elements may be configured so that each element isindividually accessible, e.g., a NOR memory array. NAND and NOR memoryconfigurations are exemplary, and memory elements may be otherwiseconfigured.

A “computer-readable medium,” “machine readable medium,”“propagated-signal” medium, and/or “signal-bearing medium” may compriseany device that includes, stores, communicates, propagates, ortransports software for use by or in connection with an instructionexecutable system, apparatus, or device. The machine-readable medium mayselectively be, but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. A non-exhaustive list of examples of amachine-readable medium would include: an electrical connection“electronic” having one or more wires, a portable magnetic or opticaldisk, a volatile memory such as a Random Access Memory “RAM”, aRead-Only Memory “ROM”, an Erasable Programmable Read-Only Memory (EPROMor Flash memory), or an optical fiber. A machine-readable medium mayalso include a tangible medium upon which software is printed, as thesoftware may be electronically stored as an image or in another format(e.g., through an optical scan), then compiled, and/or interpreted orotherwise processed. The processed medium may then be stored in acomputer and/or machine memory. In an alternative embodiment, dedicatedhardware implementations, such as application specific integratedcircuits, programmable logic arrays and other hardware devices, can beconstructed to implement one or more of the methods described herein.Applications that may include the apparatus and systems of variousembodiments can broadly include a variety of electronic and computersystems. One or more embodiments described herein may implementfunctions using two or more specific interconnected hardware modules ordevices with related control and data signals that can be communicatedbetween and through the modules, or as portions of anapplication-specific integrated circuit. Accordingly, the present systemencompasses software, firmware, and hardware implementations.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the structure of the variousembodiments. The illustrations are not intended to serve as a completedescription of all of the elements and features of apparatus and systemsthat utilize the structures or methods described herein. Many otherembodiments may be apparent to those of skill in the art upon reviewingthe disclosure. Other embodiments may be utilized and derived from thedisclosure, such that structural and logical substitutions and changesmay be made without departing from the scope of the disclosure.Additionally, the illustrations are merely representational and may notbe drawn to scale. Certain proportions within the illustrations may beexaggerated, while other proportions may be minimized. Accordingly, thedisclosure and the figures are to be regarded as illustrative ratherthan restrictive.

It is intended that the foregoing detailed description be understood asan illustration of selected forms that the invention can take and not asa definition of the invention. It is only the following claims,including all equivalents that are intended to define the scope of theclaimed invention. Finally, it should be noted that any aspect of any ofthe preferred embodiments described herein can be used alone or incombination with one another.

We claim:
 1. A method for reassigning programmed states in a multi-levelcell (MLC) memory block of at least three bits per cell, the methodcomprising: performing a first programming of original data to the MLCmemory block using a program verify for single level cell (SLC)programming; adjusting a read verify for a second programming whereinthe adjusted read verify establish a number of states that correspondwith a lower number of bits per cell than the at least three bits percell; and second programming new data using the adjusted read verify. 2.The method of claim 1 wherein a number of states corresponding with thesecond programming from the adjusted read verify is different from anumber of states associated with the at least three bits per cell. 3.The method of claim 1 further comprising: setting a programming flag toidentify when the second programming uses the adjusted read verify. 4.The method of claim 3 further comprising: receiving a read operation ofthe MLC memory block; and utilizing the adjusted read verify after thesecond programming.
 5. The method of claim 1 wherein the memory block iserased before the first programming and the second programming does notrequire erase.
 6. The method of claim 1 wherein the at least three bitsper cell comprises four bits per cell.
 7. The method of claim 6 whereinthe lower number of bits per cell comprises two bits per cell.
 8. Themethod of claim 6 wherein the lower number of bits per cell comprisesthree bits per cell.
 9. The method of claim 1 wherein the original datacorresponds with an upper page and the new data corresponds with a lowerpage.
 10. The method of claim 1 wherein the original data correspondswith a lower page and the new data corresponds with a upper page.
 11. Astorage device comprising: a memory comprising memory blocks withcircuitry for at least three bits per cell in the memory blocks; acontroller coupled with the memory that is configured to receive aprogramming request for the memory blocks; and program state circuitrythat first programs using single level cell (SLC) programming with aprogram verify level corresponding to SLC and that second programs usingthe SLC programming with a modified program verify level correspondingwith a fewer bits per cell than the at least three bits per cell. 12.The storage device of claim 11 wherein the program state circuitry ispart of the controller.
 13. The storage device of claim 11 wherein theat least three bits per cell comprises three bits per cell and the fewerbits per cell comprises two bits per cell.
 14. The storage device ofclaim 13 wherein the program state circuitry provides for four statesafter the second program, while the memory circuitry is designed forthree bits per cell and not capable of two bits per cell.
 15. Thestorage device of claim 11 wherein the memory block is erased before thefirst programming.
 16. The storage device of claim 11 wherein the memorycomprises a three-dimensional (3D) memory configuration, and wherein acontroller is associated with operation of and storing to the flashmemory.
 17. The storage device of claim 11 further comprising: a flagstored by the controller to identify that the block has a value of thebits per cell for a read operation, wherein program verify levels dependon the value of bits per cell.
 18. The storage device of claim 11wherein the second programming does not require erase.
 19. A storagedevice comprising: a memory comprising memory blocks with circuitry forfour bits per cell in the memory blocks; a controller coupled with thememory that is configured to receive a programming request for thememory blocks; and program state circuitry that first programs usingsingle level cell (SLC) programming with a program verify levelcorresponding to SLC and that second programs using the SLC programmingwith a modified program verify level corresponding with two bits percell.
 20. The storage device of claim 19 wherein the second programmingdoes not require erase.